A 20" festae would be a monster....never seen one that big but anything's possible. They typically max out at maybe 14" or so.
They're pair fish, so a 3rd wheel would typically be beaten by one or the other (or both).
The best way to get a stable pair is to grow out a group of 6 or so. A 90g would be good for that. Once you have a pair, a 6' tank's going to be necessary...and certainly not a guarantee that they won't off each other.
Matt